Drift compensated direct-current amplifier



July 4, 1950 L. w. PARKER 2,513,354

DRIFT COMPENSATED DIRECT CURRENT AMPLIFIER Filed Aug. 10, 1946 A T TOE/VF) Patented July 4, 1950 DRIFT-.YCOMPENSATED DIRECT-CURRENT I AMPLIFIER g 7 Louis W Parker, "Jackson Heights, -'N.'Y., assignor to Federal Telecommunication"Labs.," Inc.;New York; N. Y.,1a'c0rporation of Delaware Application August 1o,.ieiefserial'no.iesavse This invention relates generally to direct cur- 2 Claims. (Gal-79 171) rent amplifier "devices, and particularly to such devices which arecompensated for drift.

'It'fis wellknown'in theart-that direct current amplifiers'tendto 'be'troubled by slow drifts in LD."C. plate current, resulting from'changes in power supply voltages, or from .other causes.

Such variationsare amplified in precisely the same manner asappliedD. C. signals, since the amplifier cannot distinguish between desired and undesired voltages impressed thereon. It is, ac-

'cordingly, essential in designing D. C. amplifiers,

toprovide devices or mechanism for balancing out or Otherwiseeliminating or counter-balancing to 'themaximum extent possible the effects of such drifts.

' It is an object of the present invention to pro- I vide a drift compensated'D. C. amplifier.

It'is a furtherobject of the invention to provide a drift compensated video arnplifier device in which the gain of the device 'is modified for the D. 0. components of a video signal, without modifying the gain-for the A. 0. components of that signal.

It is still a further object ofthe invention to provide circuits for increasing the gain of a, D.'C.

amplifier device'when a voltage drift of greater I than a predetermined amount occurs in one sense and for decreasing the gain when'the "drift takes place inanother sense, whereby'to' main- Which are due-todrift of appliedvoltages, I utilize"a=constant' current device and a' thyrite sta- 'bi lizing resistancein parallel, and which are con- 'nected inseries with-e, portion "of *the coupling resistance network. Suitable voltage is applied to the thyrite from an auxiliary source-of such voltage in such manner as to causezero voltage to appear across thelthyrite during ex-is'tenceof a, normal'D. C. component at the plate =of "the first tube. I

Upon" occurrence "of moderate variations in the D. C. componentfthe action of the compensating circuit is negligible'since v'oltagedrop existing in the-series "connected portion of the res'istance network is maintained constant by the aforementioned constant current device. A' considerable 'increa'se in -the DfC. potential atw'the plate of the first tube, on "the other' hannl'results in a considerable modification of voltage across the thyr'ite 'stabilizing resistance, causing anacclerating or-non-linear change of *resist- 'ance' therein. "Thereby, a 'change'in current flow in the series connected portion of the resistance networktakes placein such'sense' as to'set up a compensatory voltage "drop therein, which serves to re'duce'the magnitude of 'theDJ'C'. variation.

By 'reason of the non linear' voltage "internal resistance characteristic of thyrite, "the 'com- 'pensatory voltage drop above referred to is of tain accurate"D. C. amplification over a pre'de- 'termined range.

It is another object of theinventionto provide drift compensation in D. C. amplifiers by means of regulating-devices which operate only in response to drifts of greater'than a' predetermined amount.

The above and stilhfurther'objects and advan- 11 'tages of the invention will become evident upon study'oi the following detaile'd'description of an embodiment thereof, taken inconjunction with the accompanying :drawings, whereinthe single figure is a schematic circuitdiagram'of an -em'- bodiment of the. invention.

In general, in the practiceof thepresent invention, a video signal 'istransferred'from the plate of one amplifier tube to' the grid of a suc and: a coupling resistance. networkwhich are con .nected. in parallel, the coupling condenser serving to' pass' the A. .C. Jcomponentsoffthevideo signal;andthei'resistance network theD. 0. component. In orderito'transfer moderate variations .of? D. C..'character 'from'the plate of the first tube, aforementioned, .to .the grid "of the succeeding :tube, while compensating for or redu'cing'itransferred voltages -of.'-.con'siderable magnitude, and

ceeding amplifiertube' over a couplingcondenser relatively small or negligible magnitudefor' moderate -D'. '0. Voltage variations in the amplifier, increasing sharply, however, as the'DnC voltage variations'become of considerable magnitude.

Referringnow to' thedrawing in detail, "the numeral *1 denotes avacuum tube wliich'is utilized 'foramplificationof a video signal which may beimpressed over'lea'd"2' between grid3' of the tribal and ground. Thecathodefl 'of tube 1 is grounded and plate potential is "applied from -'a source (not shown) "to"termina1'*-5, which is connectedvia load resistance '6 to the-plate 1.

Variations in gridpotential atfgrid Z, which may be produced by video 'signals applie'd'thereto, produce corresponding variation inpla'tecurrent, and consequentlyinthe potentials "ofthe plate 1. The latter potentials are impressed over .a coupling circuit, denominated generally by the numeral 8, to the grid QOfratube lfi, the latter further comprising at least a cathode II and a plate t2, the cathode ll'being grounded and the ;plate !2 being supplied-with D.-C.-voltage from raterminal 1.3 over a load resistancej'i. Ampli- .fied video-signalsare available, with the above arrangement, on lead I! 5, and may be utilized for any desired purpose.

Alternating current components of "the" video voltage. a I

3 tive bias.

signal applied to the grid 3 of the tube I are applied to the grid 9 of the tube Ill, after amplification by the action of the tube I, over a coupling condenser I6, which is connected between plate I and grid 9.

D. C. potentials at the plate I are transferred to the grid 9 of the tube I over resistances I! and I8 in series, the resistance I8 serving as a grid leak for the tube I0.

It will be clear from the above description, that,

'4 the part which the thyrite stabilizing resistance 29 plays in these reactions will now'be explained. If we assume a moderate variation in the D. C. component ofa video signal, the average poten- ''tial between plate I and ground. will vary similarly. Due to the constant current properties of the tube I9, however, the current flowing in resistance I! will not change, and conse- .quently the voltage drop thereacross will remain so long as tube I operates on a linear portion of its characteristic, the A. 0. component of a video signal applied to the grid 2 will be translated to the grid 9 of tube I0 without. distortion, and that variation or drift of the applied D. C. operating In order to compensate for the-drift of D. C. operating potentials applied to the tube I, I coni nect the low potential end of the resistance I1 in series with a well-screened pentode tube, I-9,rj 1

having a plate 20, a cathode 2I,control grid 22 and screen grid '23. The screen, grid 23 is operated at a negative potential which is determined by the voltage drcpexisting across the variable cathode resistance 24. The-potential of the control grid 22 is determined by its connection to a point 25 on the potentiometer 26 which is connected across a source of potential -21, the high tained substantially constant by the well known operation of the gaseous diode-28.

The bias voltages applied to the-tube I9 "are set at sufficiently high values to'ma'intain the plate current in the said tube substantially'independent of variations ofpotential at the plate 29 thereof. By variations of the value of resistanceZlthe 1 current in the tube I9 is adjusted to have a'value such that in flowing throughresistance IT, a potential drop is developed having a magnitude equal to the average voltage for the-tube 10, the grid 9 being thus maintained at a suitable nega- At the same time. stabilizing rewill be clear that under these conditions no potential will exist across stabilizing resistance 29,

and-that the amplifier will operate normally, the A. C. component of video signals reaching the grid 9 via condenser 'I6,-and the D. C. component 1 over resistances I I and I 8.

constant. The value of the potential at any point on resistance IT with respect to ground will then follow variations in the potentials of plate I with respect to ground, and these variations will be thus transferred to the grid 9 over resistance I8.

Should we assume, however, a considerable rise in the D. C. plate voltage of the tube I the consequent rise with respect to ground of the potential on plate-20 produces a considerable drop in resistance in'the thyrite stabilizing resistance 29, and a flow of currenttakes place through thyrite 29, which must be supplied over the resistance l1, increasing .the voltage drop thereacross, andconsequently decreasing the potential which would otherwise have been impressed on the grid 9.

-Should we assume, on the other hand, a considerable drop in potential at the plate I, the point of connection of the thyrite 29 to the potentiometer 26 wil1-be-ata higher potential than the plate 29; and some of the normalplate'current for the tube I9 :will be supplied over the thyrite stabilizingresistance 29, whereby the total current flow in the resistance} [and consequently the voltage drop thereacross will diminish, and a greater voltage will be transferred from the plate .I-to the grid 9-, than would otherwise be the case. i 1

- -It will be clear, then, that the action. of the thyrite stabilizing resistance and its associated circuits serves to set up-a voltage drop in the resistance I1 which modifies the voltage with respect to ground of the plate 29 in such direction as to compensate for changes in the potential of the plate I. This compensating *efiect isu zero at normal values of potential of the plate i, and increases at first slowly and then more'andmore sharply as the D. C. potential of plate I: departs farther and farther from its normal value, due

' to the non-linear voltage characteristic of the For stabilizing resistance 29, I utilize a thyrite if having a characteristic'as follows:

Resistance of Voltage across Thyrite stabilizing resistance 29. The stabilizing action does not afiect amplification of A. C. signals, since these are transferred over condenser I5, and therefore their distortionless amplification depends on linearity of tube characteristics only.

While I have disclosed a' specific embodiment of invention, it will be clear that'modifications of the specific arrangement described and illustrated maybe resorted to without departing from the spirit of the invention, which is defined in the appended claims;

What I claim and desire to secure by Letters Patent of the United States is:

1. A drift compensated amplifier comprising a first and a second source of potential, a first electron discharge tube having an output electrode connected tosaid first source, a second electron discharge tube having an "input electrode, a two-portion resistance network having a common point, said network coupling said output electrode and said input electrode for transferring direct current components, means connected between said electrodes'for transferring alternating current components, a constant-current device connected between the common point of said network and said first source, and a nonlinear resistance connected between the common point of said network and said second source whereby variations in magnitude of said first source are compensated for in an opposite sense.

2. A drift compensated amplifier as claimed in claim 1, wherein said constant-current device comprises an additional electron discharge tube having a control electrode, an anode, a cathode, and a screen grid, a potentiometer connected across said second source, the control electrode of said last-named tube connected to a point on said potentiometer for applying a negative biasing potential thereon, a cathode resistor connected between the cathode and the screen grid of said last-named tube for applying a negative 15 1 9 potential on said screen grid, a gaseous diode connected between said control gird and said screen grid, said non-linear resistance connected between the common point of said network and a point of potential on said potentiometer equal to the normal operating bias of said second tube, whereby the resistance of said non-linear resistance is maximum.

LOUIS W. PARKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Wolff Oct. 15, 1935 2,369,138 Cook Feb. 13, 1945 2,392,415 Soller Jan. 8, 1946 

